The present invention relates to an improvement in the manufacture of solid-electrolyte tantalum capacitors and is concerned more particularly with encapsulation. It is necessary to recall here the principal stages in the manufacture of tantalum capacitors such as have been described in the French Pat. No. 1,091,097 applied for by Western Electric on Nov. 25, 1953.
The anode is produced essentially from tantalum powder having well-defined grain size, by pressing followed by sintering at temperatures which depend upon the required performances of the capacitor and which are in the neighbourhood of 1800.degree. C. By means of this metallurgical treatment, a porous anodic structure is obtained, which is thereafter subjected to an anodization for the purpose of forming over the whole of the surface of the anodic sponge a layer of tantalum oxide which performs the function of the dielectric of the capacitor. The oxidised anodic structure is thereafter covered by a layer of manganese dioxide (the cathode) obtained by impregnation of the porous structure with a solution of a manganese salt which decomposes into dioxide by pyrolysis. The pyrolysis step sometimes deteriorates the oxide layer, which must be subsequently reformed. In order to obtain a dioxide layer of sufficient thickness, it is also usual to proceed with a number of successive impregnations, followed by pyrolyses and reformations. The cathode connection of the capacitor is thereafter formed by depositing a number of conductive layers on the structure thus obtained. When provided with lead wires, the capacitor is completed by an encapsulation step.
The use of metallic casings to protect the capacitors from the environment leads to high cost components.
More economical processes have been developed, in which powdered plastic materials are used to coat the components by the application of processes such as transfer moulding, fluidisided beds, etc. These processes involve bringing the component which is to be coated to a sufficient temperature to ensure thermosetting of the pulverulent plastic material with a view to obtaining a continuous flim having appropriate mechanical properties coating the component. Suitable plastic materials which are at present available polymerise at temperatures above 150.degree. C., the reaction speed increasing with the temperature. It is therefore desirable, from the industrial point of view to carry out this step in the upper temperature range, for example between 200.degree. and 250.degree. in the case of epoxy powders. Polymerisation at high temperature also affords greater reliability due to better continuity of the film. However, it is known that heating of the capacitor at a temperature of the order of 200.degree. to 250.degree. C. spoils the characteristics of the capacitor. On curve A of FIG. 1 the values of the relative capacitance variation between 25.degree. C. and 85.degree. C. as a function of the temperature at which the capacitors have been heated in the course of the encapsulation are plotted. This variation is considerable at temperatures above 200.degree. C. In order to avoid this disadvantage, it has been usual to thermoset the plastic material at lower temperatures and therefore for a longer time and with a risk of incomplete reaction.